In the chemicals and foods industries, fractional crystallization processes are regularly used to purify intermediate products and/or to isolate fractions having more desirable properties than their starting materials. These starting materials can thereto be dissolved in a solvent to provide a solution that is then subjected to a fractional crystallization process. They can also be melted whereupon the melt is then partially crystallised. During such crystallization processes, the temperature of the solution or melt is lowered causing the solution or melt to become supersaturated and generate crystal nuclei. On further cooling, these nuclei grow into macroscopic crystals and form a crystal slurry. This slurry is then subjected to a separation process which can be a continuous process using a drum filter, a continuous band filter, a sieve centrifuge or a decanter, or it can be a discontinuous separation process employing a filter press such as a plate and frame or a membrane filter press. In the fractionation of edible oils and fats, such membrane filter presses are now standard equipment (Th. Willner and K. Weber, “High-pressure dry fractionation for confectionery fat production”, Lipid Technology vol. 6, pages 56-60, 1994).
An advantage of such membrane press filters is that they leave relatively little interstitial oil between the fat crystals and thus generate a relatively pure filter cake. Other ways of achieving same, involve using a solvent, which will dilute the interstitial oil, and especially washing the filter cake with clean solvent to dilute the interstitial solution even further. So whereas for the fractionation of edible oils and fats the so-called ‘dry fractionation process’ is generally preferred since it does not involve the use of highly inflammable solvents such as acetone or hexane, some products can only be obtained by using solvents during their fractional crystallization.
In the dry fractionation process, the fat to be fractionated is melted and then cooled in a crystallizer to produce a slurry of fat crystals in its mother liquor. This slurry is then separated into a filtrate that is commonly referred to as the ‘olein fraction’ and a filter cake that is called the ‘stearin fraction’. The properties of the olein mainly depend upon the final crystallization temperature whereas the properties of the stearin fraction depend only slightly on this temperature but strongly on its olein content.
For the industrial dry fractionation of edible oils and fats, a batch crystallization process is generally preferred despite the fact that a continuous process is likely to be considerably cheaper. Such batch processes allow of better control of the crystallization process parameters such as temperature and intensity of agitation. In addition, such batch processes have the advantage over continuous processes in that the temperature of the batch crystallizer is raised above the melting point of the feedstock during each batch cycle. Accordingly, any crystal deposits on the crystallizer walls or cooling elements will be removed by being melted and no special wipers to clean these surfaces such as for instance described in U.S. Pat. No. 6,355,218 are needed.
A simple batch crystallization vessel used for the dry fractionation of edible oils and fats is the so-called “tubular crystallizer”. This is a tall, cylindrical, double-walled vessel fitted with a central, axial agitator. Heat transfer is through the vessel wall and to ascertain a cooling surface of e.g. 7 m2 per m3 crystallizer volume, the diameter of such tubular crystallizers has to be limited to 0.6 m at most. Since agitators that are longer than 5 m would require an extra sturdy execution, the volume of this type of crystallizer is in practice limited to approximately 1.4 m3; it is therefore mainly used for low-volume specialty fats, and less for bulk products such as palm oil.
For the fractionation of for example palm oil, large crystallization vessels are usually needed and various ways of providing additional cooling surface have been developed (see FIG. 11.8 at page 200 in Introduction to Fats and Oils Technology, R. D. O'Brien, W. E. Farr and P. J. Wan Eds. AOCS Press, Champaign, Ill.). These can entail cooling coils winding their way inside the crystallization vessel from bottom to top (F. Tirtiaux, “Le fractionnement industriel des corps gras par crystallization dirigée—procédé Tirtiaux” in Oléagineux (1976) 31:279-285), whereby a specially designed central agitator ensures both homogeneity of the temperature inside the vessel and prevents crystals from depositing. However, the agitating action in this construction does not extend right to the bottom of the cylindrical vessel. This may cause crystals to settle at the bottom and stay behind when the vessel is emptied. Similarly, crystals may also stay behind between the cooling coils and the vessel wall and on top of the cooling coils. This is the problem that also faces the crystallizer described for use in a batch process in Belgian Patent No. 1,005,617.
This problem has been partially overcome by fitting first vertical cooling tubes and then vertical cooling fins instead of tubes inside the vessel and so avoiding crystals to stay behind on top of the heat exchanger elements. Moreover, by directing the flow of oil towards those fins, a good heat transfer coefficient of some 70-100 W/m2K is ascertained. However, the problem of incomplete emptying of the vessel once the liquid level has dropped below the lowest agitator blade remains.
A substantial increase in cooling surface per unit volume of oil has been realised in a crystallizer comprising concentric annular crystallization compartments that are separated by concentric, annular, double-walled cooling elements. In addition, this type of crystallizer also exhibits an increased heat transfer coefficient of about 120-170 W/m2K presumably because the agitator blades move very close to the heat exchange surfaces. However, this type of crystallizer is complex in construction and therefore expensive to build. Because the agitator blades are mounted on spokes connected to a central agitator shaft and move inside the annular compartments, these must be perfectly circular to prevent the blades from scraping the walls. Besides, including a labyrinth inside the double-walled cooling elements is not easy. The problem of incomplete emptying has been solved by emptying the crystallizer as rapidly as possible into an intermediate storage vessel from where the separation equipment is being fed. This rapid emptying has the advantage that it allows more batches to be crystallised in a given time period and thus to make more effective use of an expensive piece of equipment.
Finally, a totally different solution to the problem of increasing the cooling surface per unit of volume, and especially increasing the heat transfer coefficient, is presented by the STAR-crystallizer shown in FIG. 2 of K. Weber et al. “Fat crystallizers with stirring surfaces: theory and practice” in OCL (1998) 5(5):381-384. This crystallizer is provided with an agitator that is an assembly of tubes that form a conduit for the heat exchange medium (cooling water). The assembly rotates and the rotation is eccentric. Heat transfer coefficients as high as 300 W/m2K have been measured for this type of apparatus on liquid oil.
All the prior art crystallizers described above have in common that they are provided with rotating agitators the various points of which do not move at substantially the same linear speed. Accordingly, the agitation within these crystallizers is far from uniform in that the linear velocities (speeds) of various points of the agitator vary widely according to their distance to the axis of rotation. In a cylindrical crystallization vessel with a diameter of 4 m, the agitator tip speed will be already 3 m/s when the agitator rotates 15 times per minute but close to the agitator axis, this linear speed is much lower. This lack of uniformity of linear agitation speed is believed by the present inventors to have a significantly deleterious impact upon the morphology of the crystals formed during the fractional crystallization process of oils and fats, and upon their behaviour during the subsequent separation stage.
DE 19520675A1 discloses a process for crystallizing substances by cooling of a substance-containing liquid medium in a crystallizer, in particular for crystallizing organic fats from a melt, in which within the crystallizer at least one cooling element is provided for cooling the liquid medium and in which a relative motion is generated between the medium and the cooling element, characterized in that the medium is essentially static relative to a wall of the crystallizer and that the cooling element is subdivided into a plurality of tubular heat transfer elements, which move in closed paths and that all the heat transfer elements are moved essentially with the same speed relative to the medium. DE 19520675A1 further discloses an apparatus in which the cooling unit is moved in relation to the wall of the crystallizer, through a coupling to a drive shaft and the coupling acts as a movement transmission to convert the central rotation of the drive shaft into an off-centre cooling element movement.
BE 877,839A relates to agitated heat exchangers for crystallised syrups. It discloses a vertical cylindrical vessel comprising a heat exchanger spirally arranged in a horizontal plane and to which is imparted a vertical downward and upward motion, and states that this construction allows for a better agitation than that obtained by means of a disc or a rotative blade.
DE 552,532 discloses a crystallization vessel wherein a cooling hollow body and a drive are suspended. The drive imparts to the cooling hollow body a jerky vertical motion or a pendular motion around an axis in a horizontal direction.
GB 2,100,613A discloses producing a metallic slurry in which a metallic material is agitated by a reciprocating agitator. For instance the agitator is a plate or disc and is reciprocated in a direction substantially perpendicular to its major plane.
JP 07-284,643A discloses an agitated vessel for fishery product, said vessel comprising a plate impeller freely detachably hung from a rectilinearly reciprocating slider, wherein the impeller is inclined to the moving direction of said slider. JP 2002-210,399A teaches preventing sedimentation of a solid content in a coating liquid filled in a stirred tank by horizontally reciprocally moving a water-feed panel arranged within said tank, said panel comprising a bent blade material bent to a V-shaped cross section and a bent blade material bent to a reverse V-shaped cross section; in this embodiment, the agitating blades are inclined to the moving direction.
GB 1,424,049 discloses an apparatus suitable for carrying out transformation processes of a non-metallic substance from liquid to solid phase, such as crystallization, comprising a vertically extending hollow reaction chamber having a horizontal cross-sectional shape in the form of a polygon.
DE552532 describes a crystallizer comprising an agitator made of cooling elements moving jerkily back and forth either vertically or horizontally.